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Measure relative humidity using an infrared thermometer

While consumer digital thermometers tend to give reasonably accurate temperature readings to within 1C of the actual temperature, hygrometers can vary a lot and I don’t mean by just a percentage or two. I placed 5 digital hygrometers in the same room and left them to adapt over a few hours. To my surprise, the relative humidity percentage readings varied from 48% to 70%! The temperature readings on the other hand varied no more than 1C of each other. With three hydrometers giving similar readings (two with readings in the high 60’s and one showing 70%), I figured that the two giving the low readings were probably way off, considering it did feel quite stuffy in the room.

After reading how early relative humidity measurements at weather stations were made using wet and dry bulb thermometer readings and a calculation or graph look-up to get the RH reading, I realised that I could do this same thing to get an indication to what the relative humidity reading really is and which hygrometer was giving the most accurate reading. Instead of using a wet/dry thermometer set up, I used an IR thermometer to get the “dry bulb” temperature by pointing at some background object (e.g. a cabinet) and a web bulb temperature by pointing it at a moist cloth left hanging for several minutes. Then I use an online calculator to convert these readings, along with the absolute atmospheric pressure reading into the relative humidity percentage.

How this measurement process works:

Start by rinsing a cloth in cold water and wringing it out such that it is most, but not dripping. Hang it for at least 10 minutes away from an outer wall or any heat source (which would affect its temperature). For example, hanging on the knob of a wooden cabinet should be adequate. During this period, like the moist sock on a wet bulb thermometer, the water slowly evaporates from the cloth, bringing down its temperature until it settles depending on the room’s humidity level.

Now get an Infrared thermometer and scan it across the cloth a few times to record the lowest temperature shown on the display. This gives the “wet bulb” temperature, which is roughly the same as what an actual wet bulb thermometer should show. The following is an example from a moist cloth I hung up:

Next, measure the temperature of an area next to the cloth to get the “dry bulb” temperature to get the actual room temperature. Since different thermometers can give different readings, the infrared thermometer should be used for both readings to avoid a skewed result. The following is an example from the cabinet surface next to the cloth:

For the final reading, get the absolute atmospheric pressure reading, such as from a barometer. For those near sea level, the current weather’s pressure reading (e.g. weather.com) will do. If this reading is not available, a value of 1013 will generally suffice, but could leading to the percentage being a percentage or two off if the atmospheric pressure is much lower or higher.

Finally, enter the three values into this online humidity calculator: Put the readings in the respective “Dry Bulb”, “Wet Bulb” and “Atmospheric Pressure” fields and click “Compute” as highlighted in red below. The “Relative Humidity” reading will show the result. The following shows the result from the measurements in my example, as highlighted in green:

If done properly with a reasonably accurate Infrared thermometer, the readings will be more accurate than most consumer hygrometers.

For this particular hydrometer, the relative humidity was within 2%:

The reason the indoor and outdoor temperature show the same readings here is that I currently have the outdoor probe inside and right next to this weather station.

For a quick relative humidify measurement using the infrared thermometer:

Even if this hygrometer appears to show a pretty accurate reading, another issue with digital hygrometers is that they can take 30 minutes to an hour to adapt when moved from room to room, especially if the room is a significantly different temperature to the last location. So a workaround is to use the above infrared thermometer method, but instead of hanging the moist cloth, gently swing it around for 30 seconds and measure its lowest temperature reading using the infrared thermometer. Now gently swing around the cloth again for another 30 seconds and measure its lowest temperature again. Repeat this process until the lowest temperature measurement remains the same as the previous measurement.

Finally, get the room temperature by pointing the thermometer at a cabinet or any table/desk height surface other than a wall or heat source (e.g. heater, television, etc.) and perform the calculation here.